A tomato B‐box protein SlBBX20 modulates carotenoid biosynthesis by directly activating PHYTOENE SYNTHASE 1, and is targeted for 26S proteasome‐mediated degradation

Abstract

Carotenoids play important roles in many biological processes, such as light harvesting, photoprotection and visual attraction in plants. However, the regulation of carotenoid biosynthesis is still not fully understood. Here, we demonstrate that SlBBX20, a B-box (BBX) zinc-finger transcription factor, is a positive regulator of carotenoid accumulation in tomato (Solanum lycopersicum). Overexpression of SlBBX20 leads to dark green fruits and leaves and higher levels of carotenoids relative to the wild-type. Interactions between SlBBX20 and DE-ETIOLATED 1 (SlDET1) lead to the ubiquitination and 26S proteasome-mediated degradation of SlBBX20. Moreover, deficiencies in the components of the CUL4-DDB1-DET1 complex enhanced the stability of the SlBBX20 protein. Thus, we conclude that SlBBX20 is a substrate of the CUL4-DDB1-DET1 E3 ligase. SlBBX20 can activate the expression of PHYTOENE SYNTHASE 1, encoding a key enzyme in carotenoid biosynthesis, by directly binding to a G-box motif in its promoter, which results in the elevated levels of carotenoids in SlBBX20 overexpression lines. We identified a key regulator of carotenoid biosynthesis and demonstrated that the stability of SlBBX20 is regulated by ubiquitination. These findings provide us a new target for the genetic improvement of the nutritional quality of tomato fruit.

ZmPBF and ZmGAMYB transcription factors independently transactivate the promoter of the maize (Zea mays) β‐carotene hydroxylase 2 gene

“…Very little is known about the regulation of these genes in cereals, or indeed in other plants. A few transcription factors have been shown to influence carotenogenic genes in dicots, including Arabidopsis RAP2.2 and PIF1 (Welsch et al ., ; Toledo‐Ortiz et al ., ), tomato ( Solanum lycopersicum ) RIN and BBX20 (Martel et al ., ; Xiong et al ., ), and citrus CrMYB68 and CsMADS6 (Zhu et al ., ; Lu et al ., ). RAP2.2, a member of the AP2 gene family, binds to the Arabidopsis PSY promoter and represses PSY and PDS (encoding phytoene desaturase) in root‐derived callus, thus reducing the carotenoid content (Welsch et al ., ).…”

“…The MADS-box protein RIPENING INHIBITOR (RIN) in tomato directly induces the expression of SlPSY1 (Martel et al, 2011). SlBBX20, a B-box (BBX) zinc-finger transcription factor, can activate the expression of SlPSY1 by directly binding to a G-box motif in its promoter, which results in the elevated levels of carotenoids in SlBBX20 overexpression lines (Xiong et al, 2018). CrMYB68, an R2R3-MYB transcription factor isolated from Green Ougan (MT), a stay-green mutant of Citrus reticulate cv Suavissima, can bind to the promoters of both CrBCH2 and CrNCED5 (9-cisepoxycarotenoid dioxygenase 5) and significantly inhibits their expression (Zhu et al, 2017b).…”

New insight into the molecular mechanism of colour differentiation among floral segments in orchids

“…Finally, the extract residues were filtered through a 0.22-μm membrane filter and stored in brown bottles. Carotenoid quantification in the samples was performed according to a previously described method 50 . In addition, the total carotenoids and anthocyanins of the samples showing transient overexpression were extracted and quantified as previously described 48,51 .…”

Changing Form and Function through Carotenoids and Synthetic Biology

“…There are other examples where crop domestication has led to loss of certain biochemical features, such as apocarotenoids no longer detected in tomato (Solanum lycopersicum) as compared to wild species (Tieman et al, 2017). A number of transcription factors that modulate transcriptional levels of the pathway structural genes have been identified (Welsch et al, 2007;Ampomah-Dwamena et al, 2018;Lu et al, 2018;Xiong et al, 2019). However, these transcription factors coregulate many other noncarotenogenic genes to coordinate networks related to carotenoid function (e.g.…”